1. Field of the Invention
This invention relates to gas-liquid contacting devices and the use of such devices in wastewater treatment. The invention especially relates to methods and apparatuses for aerating and pumping in activated sludge processes, particularly when conducted in oxidation ditches of racetrack or loop channel configuration.
2. Review of the Prior Art
Many liquid waste treatment processes, commonly termed aerobic processes, supply bacteria and other microorganisms with dissolved oxygen for treating aqueous wastes such as municipal sewage, tannery wastes, dairy wastes, meat-processing wastes, and the like.
One such aerobic process is the activated sludge process, in which the microorganisms are concentrated as an activated sludge to be mixed with incoming wastewater, which supplies food for the organisms. The apparatuses in which the activated sludge process is conducted comprise an aeration basin (reactor basin) and a final clarifier (settling tank). The aeration basin serves as a culturing basin in which to generate the growth of bacteria, protozoa, and other types of microorganisms, so that they can consume the pollutants in the raw waste entering the basin by converting the pollutants into energy, carbon dioxide, water, and cells (biomass).
The activated sludge process is effective for controlling this conversion activity within the aeration basin, for settling the biomass within the clarifier, for overflowing the purified liquor or effluent from the clarifier to discharge, and for returning the settled biomass from the clarifier to the aeration basin. Thus the activated sludge process is a suspended-growth, aerobic, biological treatment process, using an aeration basin and a settling tank, which is capable of producing very pure, high quality effluent, as long as the biomass settles properly.
It can thus be compared to a fixed-growth process wherein the growth of the biomass occurs on or within a tower on plastic media or in a trickling filter on rocks packed therewithin.
Objectives of aerobic wastewater treatments with the activated sludge process include removal of biochemical oxygen demand (BOD), phosphorus removal, NH.sub.3 -Nitrogen removal (preferably by nitrification, in which NH.sub.3 is converted to NO.sub.2 and NO.sub.3 ions), NO.sub.3 -Nitrogen removal (preferably by denitrification, in which NO.sub.2 and NO.sub.3 ions are utilized as oxygen sources and the incoming waste as a carbon and energy source), minimum usage of chemicals and energy, and minimum manpower requirements.
The activated sludge process is represented by two prime mixing regimes, plug flow and complete mixing, which represent the opposite extremes of a continuum and almost infinite variety of intermediate mixing modes.
Plug flow mixing characteristically occurs in a long, narrow aeration basin into which wastewater and return sludge are fed at the inlet end to form a dilute mixed liquor which flows toward the outlet end while oxygen is being introduced and the biomass is rapidly increasing. At the outlet end, the mixed liquor passes to a clarifier. A portion of the recovered sludge, as clarifier underflow, is pumped to the inlet end. Plug flow systems are characterized by gradients in dissolved oxygen(D.O.) and oxygen uptake rate of their mixed liquor which is inherently dominated volumetrically by the inflowing wastewater so that temporary or cyclic variations in wastewater characteristics, such as unusually large quantities of materials poisonous to microorganisms, can cause shock loadings that can at least temporarily inactivate the system. There are many hybrid systems (semi-plug flow) in which the waste and/or the sludge are admitted at intervals along the channel.
Complete mixing is commonly conducted in a round or square tank or basin into which incoming wastes are fed at numerous places while the contents are being vigorously mixed and aerated so that the wastes are rapidly dispersed throughout the tank. The volume of mixed liquor is so much greater than the volume of the incoming wastewater that the mixed liquor overwhelmingly dominates the wastewater.
Thus there is a relatively uniform food/microorganism ratio existing in such complete-mix tanks. Also, there is a uniform concentration of mixed liquor suspended solids (MLSS) to be found in complete mix aeration tanks as contrasted with the variable concentration noted in the plug flow and semi-plug flow tanks.
Both plug flow mixing (with respect to its food supply and dissolved oxygen content) and complete mixing (with respect to all added materials at the points of introduction) additionally occur cyclically within single-stage systems, commonly termed oxidation ditches, having an endless channel within which the numerous species of microorganisms in its mixed liquor go through breathing, feeding, and resting cycles each 3-10 minutes, depending upon channel length, temperature, flow velocity, food supply, and the like. The biomass in the mixed liquor is maintained at a high level of abundancy (such as 3,000-6,000 mg/l), so that oxygen tends to be quickly consumed within the aerobic portion of the channel.
These microorganisms in circuit flow within the endless channels of oxidation ditches can also be supplied with sufficient oxygen within the aerobic portion that nitrite and nitrate ions can be formed from ammonia, which is derived from broken-down proteins, and then can be sufficiently deprived of oxygen during a second portion of the cyclic flow that the nitrite ions and nitrate ions and/or sulfate ions are used as their sources of oxygen by certain species of microorganisms, provided that a carbon-supplying food source, such as methanol or incoming wastewater, is also available. This process causes nitrogen to be liberated from the mixed liquor as bubbles of gas and is termed denitrification.
However, if temperature, biomass concentration, food supply, oxygen supply, and the like should change so that the aerobic portion of the endless channel is increased in length at the expense of the anoxic portion thereof (within which denitrification occurs), denitrification can continue while the withdrawn mixed liquor is being clarified. The unfortunate result is that nitrogen bubbles can rise within the clarifier and seriously interfere with settling of the biomass to form sludge and clarified liquor.
Aerators used in activated sludge systems include bubble diffusers, mechanical surface aerators (both high speed and low speed), submerged turbine aerators, horizontal rotors, and gas-liquid jet aerators, such as eddy jets. The horizontal rotors may be fixed, adjustable in height, or floating and may be fitted with brushes, blades, cages or discs. "Submerged turbine aerator" is a term used in the wastewater treatment industry to describe a mixing device which comprises one or more axial-flow propellers or radial-flow impellers, in which compressed air or high-purity oxygen is diffused through a sparge device, located downstream of the propellers or impellers, which are attached to a vertically disposed shaft within a vertically disposed discharge or intake duct.
Only vertically mounted low-speed mechanical surface aerators, horizontally mounted rotor aerators with brushes, blades, cages, or discs, and jet aerators had been usable in oxidation ditches until the invention of the barrier oxidation ditch, as disclosed in Ser. No. 649,995, filed Jan. 19, 1976 and now abandoned, and in Ser. No. 848,705, filed Nov. 4, 1977 and now abandoned, both of which describe means for mounting and utilizing submerged turbine aerators within a deep oxidation ditch having a barrier athwart the channel, and both of which are incorporated herein by reference.
It is pertinent to note that a conventional circuit-flow oxidation ditch of the prior art operates as a complete mix system except that its D.O. gradient is characteristically plug flow. Circulation of the entire basin contents during each cycle, while admixing the mixed liquor with the relatively minor stream of inflowing wastewater, ensures such complete-mix conditions.
Although it could be stretched out so that its racetrack or looped channel would be a mile in length, for example, whereby the circuit flow in its channel would be comparable to that of the inflowing wastewater in volume, such as 1:1 to 3:1, thereby simulating a true plug flow activated-sludge system, it would then be subject to shock-load effects, the food-to-microorganism ratio in the waste inlet portion being so high that the microorganisms could readily be overwhelmed by incoming poisons or other changes in the food situation. Preferably, therefore, an oxidation ditch is sufficiently short that its channel flow of mixed liquor is ample to dilute the inflowing wastewater by volume ratios of 100:1 to 200:1 or greater, whereby the inflowing wastewater is completely dominated volumetrically by the mixed liquor in the ditch and the food-to-microorganism ratio is low enough that the microorganisms can handle any reasonable change in food properties, thereby simulating a true complete mix system.
At such desirable volume ratios, an oxidation ditch can be designed to operate with recycled sludge within its channel on a food-to-microorganism ratio (F/M) by weight that varies over a possible range of 0.01 to 5.0, depending upon space, cost, and process design requirements, by varying the concentration of microorganisms, expressed as mixed liquor suspended solids (MLSS), flowing within its channel. If operating at a low F/M ratio of 0.01-0.2, it is an extended aeration system, producing small quantities of sludge. If operating at a medium F/M ratio of 0.2-0.5, it is a conventional system. If operating at a high F/M ratio of 0.5-2.5, it is a high-rate activated sludge system, producing large quantites of sludge. Moreover, it can even be operated as an aerated lagoon with no recycle sludge at F/M ratios above 2.5, but it is then not operating according to the activated sludge process and is, therefore, not herein defined as an oxidation ditch.
An oxidation ditch may also shift through a wide F/M range, representing all three of these systems, as it begins operation as a high-rate activated sludge system, with no built-up sludge, and gradually builds up its recycled sludge to a mixed liquor suspended solids (MLSS) content of 3,000 mg/l where extended aeration can generally be considered to begin. In general, an oxidation ditch is considered for design purposes to exist when the MLSS content reaches about 1500 mg/l, because at lower levels the size of the ditch would have to be excessive, but the principles of its operation are nevertheless applicable at much lower MLSS levels, such as at 1,000 mg/l.
It is significant that increasing the concentration of the microorganisms increases the total amount of oxygen used in an oxidation ditch of given volume and necessitates a higher flow velocity to maintain the greater mass of solids in suspension. At a given rate of food inflow (F), increasing the concentration (M) of microorganisms obviously decreases the F/M ratio. A change in the F/M ratio also affects the O.sub.2 transfer rate (measured as pounds of oxygen per hour at process conditions) for which the ditch must be designed, as is known in the art. For example, using F/M to represent food content as pounds of five-day biochemical oxygen demand, BOD(5), per pound of microorganisms, O/F to represent pounds of oxygen per pound of BOD(5), and O/M to represent pounds of oxygen per pound of microorganisms, the following approximate relationships are known in the art:
______________________________________ Excess bio- Type of logical solids Typical activated Sludge (cells) produced MLSS sludge age, per lb. BOD(5) content process days applied mg/l F/M O/F O/M ______________________________________ High 0.5-2 &gt;1 500-1000 1.0 0.7 0.7 rate load Conven- &gt;2 1 &gt; 0.35 &gt;1000 0.3 1.0 0.3 tional &lt;6 &lt;3000 load Extended &gt;6 &lt;0.35 &gt;0.2 &gt;3000 0.1 1.2 0.12 aeration &lt;20 &lt;5000 Low load &gt;12 &lt;0.2 &gt;3000 0.05 1.5 0.08 extended aeration (typical for oxi- dation ditch) ______________________________________
In order to remove nitrogen from a wastewater, in which it may be measured as total nitrogen or total Kjeldahl nitrogen, all systems using the wastewater as the chief organic carbon source for denitrification employ an alternating aerobic-anoxic sequence of stages, without intermediate clarification, to effect total nitrogen removal while attempting to avoid ammonia nitrogen bleedthrough. An oxidation ditch can be used for this purpose by controlling the level of aeration so that the mixed liquor is recirculated many times through alternating aerobic and anoxic zones prior to discharge from the channel of the ditch. To operate effectively, however, it is important that both zones be uninterrupted; i.e., aeration should occur at a single location immediately preceding the aerobic zone and should not recur until at least the end of the anoxic zone.
If aeration occurs at only one location, so that there follows downstream thereafter one and only one aerobic zone, one and only one anoxic zone, and, if desired, an oxygen-deficient zone within the channel of the ditch, it is herein defined as "point-source aeration". If there are multiple zones of each type, it is defined as "multi-source aeration".
"Point-source aeration", "point-source mixing", and "point-source propulsion" are terms signifying that these three properties (hereinafter generally termed "point-source treatment") each originate at a single location within the channel of an oxidation ditch, in contrast to multiple locations therefor.
A precursor of the oxidation ditch is described in U.S. Pat. No. 1,247,542 of Jones (1917). It has a racetrack-shaped channel, a plurality of baffles disposed across the channel, and air diffusers on the downstream side of each baffle, thereby creating eddy-type flow between nearby baffles.
A system that is superficially similar in plan view is shown in U.S. Pat. No. 1,643,273 of Imhoff (1927). It comprises a pond in which fish may be raised, a ditch which is connected to the pond at each end, an inlet to the ditch for sewage which has passed through a settling basin, and a pump device in the ditch for withdrawing clean water from the pond so that it may dilute the settled sewage before it is fed to the pond. The pump device comprises a baffle which is disposed athwart the ditch, a passage beneath the baffle, and an airlift pump alongside the baffle so that its operation pulls the water through the passage. It does not utilize the activated sludge process because there is no mixed liquor in the ditch. Most of the oxygen transfer occurs by algae activity in the pond, and the intake D.O. content is generally high so that the pump is not intended to function primarily as an O.sub.2 transfer device. In addition, because the pond itself functions as a barrier, there is no induced flow approaching the pump.
The first oxidation ditch was invented by Dr. Ir. A. Pasveer and is described in Netherlands Pat. No. 87,500 and British Pat. No. 796,438 (1958). The mixed liquor in its racetrack-shaped channel is aerated and propelled by a horizontally disposed brush-type rotor, which acts as a booster pump for the mixed liquor flowing therebeneath. Clarifying is performed intermittently within the channel.
An oxidation ditch of conventional shape, with a post-sedimentation reservoir or clarifier therewithin for sewage treatment is described in U.S. Pat. No. 3,421,626 of Schramm et al (1969). Two rotors are operated within the endless channel during dry weather. When excessive rainfall occurs, the rotors are stopped, and the channel of the oxidation ditch becomes a preliminary settling basin.
An oxidation ditch in which the mixed liquor is agitated, aerated, and propelled by a shrouded drum having rotor blades is described in U.S. Pat. No. 3,452,873 of Blough (1969) for disposing of livestock manure.
The Pasveer oxidaticn ditch was improved in Holland by deepening the ditch, separating the straight reaches of its endless channel with a thin partition, and installing a vertically mounted, low-speed mechanical surface aerator at an end of the partition, as disclosed in U.S. Pat. No. 3,510,110 of Klein (1970). Depths of up to 16 feet are feasible. A complete mix zone is created around the aerator, and booster-type pumping occurs when a portion of the toroidal flow strikes the partition and is converted into a slowly spiralling downstream flow. Clarification occurs continuously in a separate vessel, with a portion of the sludge being returned to the oxidation ditch. This system is marketed under the trademark "Carrousel".
A multichannel oxidation ditch, in which discs mounted on a common shaft act as pump/aerators for sewage treatment, is described in U.S. Pat. No. 3,579,439 of Meiring et al (1971). The mixed liquor flows sequentially from one concentrically disposed channel to the other. The channels can be operated for nitrification and denitrification by controlling the amount of flow between the channels and the settling tanks.
An inverted funnel is suggested in U.S. Pat. No. 3,643,403 by Speece (1972) for aerating oxygen deficient water by countercurrent flow with ascending air bubbles.
An oxidation trench having a longitudinally disposed partition and a vertical discharge duct in the middle of the partition or at one or both of its ends, with the inlet of the duct being at the bottom of the partition and at one side thereof and the discharge duct and outlet at the other side, is described in German Patent No. 2,300,273 (1973). A mechanical surface aerator is vertically mounted within the outlet. It provides merely one half of its propulsive force for use in the discharge channel in each direction if its pump/aerator is disposed in the middle of the partition. It is, moreover, subject to short circuiting or backmixing if the pump/aerator is nearer to one end.
An elongated oxidation ditch of looped-channel configuration, in which the mixed liquor is circulated directly through the cooling condensers of an electric power generating plant by a single rotor equipped with discs, is described in U.S. Pat. No. 3,760,946 of Boler (1973).
An oxidation ditch of the looped-channel type which is used as a channeled algae pond, in which the liquid is propelled and agitated by a plurality of horizontally disposed cage aerators or aeration brushes, is described in U.S. Pat. No. 3,839,198 of Shelef (1974).
Circular and elongated oxidation ditches in which the mixed liquor is aerated and propelled by banks of jet ejectors are described in U.S. Pat. Nos. 3,846,292 of Lecompte, Jr. (1974); 3,897,000 of Mandt (1975); and 4,199,452 of Mandt (1980).
A denitrification process for use in oxidation ditches having a vertically mounted mechanical surface aerator at one or both ends of a longitudinally disposed partition is described in U.S. Pat. No. 3,900,394 of Rongved (1975).
An oxidation ditch utilizing the aeration rotor of U.S. Pat. No. 3,759,495 of Boler et al (1973) is described in U.S. Pat. No. 3,905,904 of Cherne et al (1975).
An oxidation ditch which includes a bank of venturi-type ejectors is described in U.S. Pat. No. 3,990,974 of Sullins (1976). A dissolved oxygen sensor controls pumping of liquor through the ejectors and thereby the quantity of air sucked from above the liquor level into the throats of the ejectors. Banks of vertical settler tubes are also disposed at one end of the oxidation ditch to effect settling of particles after impingement upon the inside surfaces of the tubes.
An oxidation ditch having a transversely disposed barrier across the channel is described in Hungarian Pat. No. 166,160 (1976) and Austrian Pat. No. 339,224 (1977), the barrier being traversed by a discharge duct containing a smaller duct within which an axial-flow impeller is disposed, whereby the smaller duct discharges within the larger one and functions as an upwardly discharging jet ejector.
A pair of oxidation troughs, each equipped with a drum-type rotor, is connected in series with (1) an aerated buffer vessel to receive incoming waste, (2) an aerated high-capacity tank for mixing the pre-aerated waste with return sludge and for feeding the pair of oxidation troughs and (3) a clarifier for receiving effluent from the troughs, as is described in U.S. Pat. No. 4,138,328 of Schnitzler (1979).
An oxidation ditch utilizing surface aerators is disclosed in U.S. Pat. No. 4,159,243 of O'Key (1979). The disclosed oxygen concentration is measured at two or more points in the endless channel and the aerators are controlled accordingly to produce desired lengths of oxic and anoxic zones.
An oxidation ditch is taught in U.S. Pat. No. 4,269,709 of Rongved (1981). It comprises a surface aerator adjacent one end of a dividing wall and a tilted partition attached to the same end for deflecting and accelerating the flow of liquor, whereby damage to flocs by destructive turbulence is minimized.
An oxidation ditch utilizing rotors for surface aeration of its mixed liquor is described in U.S. Pat. No. 4,285,818 of Muskat (1981). One rotor operates within a gas-tight cover having selectively operated flaps which are closed while oxygen is admitted after dissolved-oxygen probes have detected an undesirably low level of dissolved oxygen within the channel.
An oxidation ditch utilizing a surface aerator and having a clarifier disposed in the endless channel, so that the mixed liquor flows therebeneath, is described in U.S. Pat. No. 4,303,516 of Stensel et al (1981).
As of 1975, there were more than 500 municipal oxidation ditch installations of the horizontal rotor type in the United States and 90 in Canada, and there were 154 Carrousel installations in the world, according to "A Comparison of Oxidation Ditch Plants to Competing Processes for Secondary and Advanced Treatment of Municipal Wastes", by W. F. Ettlich, EPA-600/2-78-051, March 1978, National Technical Information Service, Springfield, Va., 22161. In this publication, these oxidation ditch plants are stated to provide flexibility in operation, a stable sludge, and performance above the average of all other competing secondary processes. Oxidation ditch plants were also found to be very competitive in operation and maintenance cost and to provide nitrogen removal at no additional cost.
However, these prior art oxidation ditches have many design and operational problems. Except for the jet ejector types, the pump/aerator devices of all prior art oxidation ditches produce spray and mist which create slippery walkways, because of algae growth in summer and freezing in winter, and also cause excessive ice formation on the aeration equipment in winter. Enhancing the surface area of liquor exposed to cold air further causes a loss of heat from the system and a reduction in reaction rates.
It is conventional practice in prior art oxidation ditches that their pump/aerators furiously aerate a portion of their liquor, while allowing the remainder to flow past untouched, and then the aerated and unaerated portions of the mixed liquor blend somewhere downstream of the pump/aerators to produce the desired dissolved oxygen content. From a hydraulic viewpoint, this practice can be termed "booster pumping", because the pump merely accelerates or adds energy to the mass of liquor flowing past the pump.
Such booster pumping seems to have developed because designers of prior art oxidation ditches have apparently believed that the kinetic energy in the induced-flow liquor is an asset that should not be interfered with. They have accordingly designed their ditches for booster pumping with single devices that combine the functions of pumping and aerating, whereby the momentum of the flowing liquor is merely augmented with each circuit-flow movement past the pump/aerator. Because the pumping function requires a relatively small input of energy, the principal capability of these devices is the aerating function. However pumping and aerating functions cannot be utilized independently.
In consequence, a multi-component price has had to be paid for this value judgment as to the importance of kinetic energy. These price components can be enumerated as follows:
(a) Backpumping of some liquor occurs from the discharge side to the intake side of the pump/aerator, so that a portion of the liquor must be pumped more than once. PA1 (b) Backmixing of an aerated portion of the backpumped liquor also occurs. The result is that this portion of the backpumped liquor which contains relatively high levels of dissolved oxygen must be aerated again and at a lower oxygen transfer rate. In addition, because prior art systems are designed to aerate a fraction of the flow rather than all of it, when the backmixed liquor is being aerated a second time, the already aerated backmixed liquor is displacing an equal volume which cannot be aerated the first time. PA1 (c) Heterogeneous aeration occurs when unaerated induced-flow liquor blends with highly aerated liquor to produce a blended downstream liquor having a desired average dissolved-oxygen content. Such heterogeneous aeration requires more energy than homogeneous aeration to the same dissolved-oxygen content. PA1 (d) Propulsion limitations occur when a portion of the liquor bypasses the pump/aerator, without being contacted or even directly influenced by the device, and is instead induced to flow by momentum. This induced-flow liquor is susceptible to being retarded or held back whenever the backpressure or frictional head becomes excessive. As a practical matter, such excessive friction is likely whenever attempting to operate an oxidation ditch at very high levels of MLSS (.apprxeq.10,000 mg/l or ppm), for the higher the mixed liquor solids level, the higher the density and viscosity. PA1 (e) Inflexibility of operation occurs because the aerating and pumping functions of the pump/aerator are performed simultaneously by the same prior art device, whereby changing the submergence or the rotational speed of a pump/aerator simultaneously affects both aerating and pumping functions. PA1 C=initial oxygen concentration, PA1 t=time, PA1 C.sub.s =saturation concentration of oxygen, and PA1 K=the overall gas mass transfer coefficient (time -1); it is a function of the resistance of the films and the area of liquid-gas interface per unit volume of liquid. PA1 (1) the air-liquid jet aerator flow, PA1 (2) the directly pumped flow, and PA1 (3) the upwardly moving mixture of liquor and diffused air from the removable contact duct diffusers. PA1 A. a deep oxygen contact duct which has an upper end in flow connection with the intake channel and is disposed to pass beneath the barrier at a selected maximum depth; PA1 B. a pump which comprises: PA1 C. an induced-flow means which comprises an annular inlet which is disposed around the central inlet; PA1 D. an inlet aeration means which comprises at least one of the following: PA1 E. an air compressing means and an air supply means which are in flow connection and are connected to the inlet aeration means. PA1 (1) at least one air/liquor header which is disposed within the annular inlet, PA1 (2) a plurality of eddy jet aerators which are flow connected to the header or headers and are disposed to discharge downwardly, and PA1 (3) a liquor pump which is flow connected to the air/liquor header or headers.
The inability to prevent backpumping of liquor from the discharge side toward the intake side of the aerator imposes a slight additional energy demand upon the system.
The inability to prevent backmixing of aerated mixed liquor with unaerated liquor is believed to be more serious. Backmixing causes smaller quantities of oxygen to be transferred into the mixed liquor for a specific exposure of mixed liquor to air. Therefore, additional energy is required to attain a desired D.O. content because oxygen is a slightly soluble gas in water and the necessary driving force increases non-linearly as the dissolved-oxygen content increases, according to the equation: EQU dC/dt=K(C.sub.s -C)
where, at the temperature of oxygen transfer:
As may be appreciated from FIG. 1, the rate of oxygen transfer, from bubbles of an oxygen-containing gas, such as air, to water, is a tangent, dC/dt, to the solubility curve plotted from this equation (for initially deaerated water at 4.degree. C. and an atmospheric pressure of 14.54 psi) at any time, t. If unaerated water is aerated, the initial slope is quite steep, such as line A in FIG. 1 (equalling 4.0 in the units as shown). If backmixing occurs so that a mixture of aerated and unaerated water is aerated, the slope is much shallower, such as line B in FIG. 1 (equalling 1.1). If heavily aerated water reaches the aeration device, the slope can be very shallow, such as line C in FIG. 1 (equalling 0.53). It should, therefore, be quite clear that, with a given input of energy, backmixing will cause a considerably smaller quantity of oxygen to be transferred into a parcel of water, as compared to the situation for aeration of unaerated water. Designers of prior art oxidation ditches, complete mix systems, and plug flow systems appear to have ignored the high cost of backmixing and even its very existence.
The inability of prior art oxidation ditches to prevent heterogeneous aeration, which occurs when highly aerated mixed liquor is blended with unaerated liquor, is also believed to be important. As can be appreciated by a glance at FIG. 1, when a quantity of aerated liquor having an oxygen concentration of 13.6 mg/l and an oxygen transfer rate C, after cumulative aerating for 10 minutes, is blended with an equal quantity having an oxygen concentration of zero and an oxygen transfer rate A, at zero aerating time, to produce a mixture having an oxygen content of 6.8 mg/l and an average cumulative oxygen transfer rate D (equalling 1.9), an average cumulative time of 5.0 minutes is spent on the entire blend, i.e., more energy is expended than would be required for homogeneously aerating up to slope D at cumulative time (equalling 2.7 minutes). Expressed in other terms, aerating the entire quantity at 0.0 starting D.O. for 5.0 minutes imparts 9.9 mg/l to the liquid. The difference between 9.9 and 6.8 represents a significant energy wastage when using the curve shown in FIG. 1 which is characteristic of one type of pump/aerator, as an example.
The practical effect in a prior art oxidation ditch of both backmixing and heterogeneous aeration, in combination, may be appreciated by considering FIG. 2. The upper portion of this schematic flow chart relates to backmixing of aerated liquor to the aerator, and the lower portion relates to heterogeneous aeration which occurs when unaerated liquor flows past the aerator and blends with aerated liquor somewhere downstream of the aerator. The combined effect of these prior art process characteristics is to force the final aeration time, t, to shift a significant distance along the aeration curve for the liquor, as illustrated by moving from the relatively steep tangent D to the shallower tangent B or even C in FIG. 1.
Propulsion limitations are often interrelated with equipment characteristics. For example, the vertically mounted low-speed mechanical surface aerators used in the Carrousel system cannot be mounted side by side and cannot be built to a size greater than 150 horsepower. In consequence, these pump/aerators must be mounted within a relatively narrow channel so that circulation capacity of the channel is limited. They are further limited as to deliverable head because liquor that is near the outer wall and near the bottom of the channel is not pumped but is induced to flow by momentum and by proximity to the pumped liquor. This induced-flow liquor is, therefore, suseptible to being retarded or held back if the frictional head is excessive. In consequence, the Carrousel system has utilized a plurality of mechanical surface aerators at ends of partitions in a looped channel configuration, operating as booster pumps in series, for large oxidation ditches. The Carrousel system is presently marketed in the United States by Envirotech Corporation.
The horizontal-rotor oxidation ditches, equipped with blades, brushes, or cages, have difficulties with stratification at depths greater than seven feet. These difficulties include settling of suspended solides, oxygen depletion, and anaerobic digestion in bottom areas, unless a slanted baffle is mounted downstream from the rotor and athwart the channel to force pumped and aerated liquor toward the bottom. Although the baffle creates mixing at lower depths, the unaerated flow beneath the rotors is nevertheless induced flow, not pumped flow, and is subject to retardation if the friction head is excessive. However, such horizontal-rotor ditches have a usual depth of 3-7 feet and a maximum depth of 12 feet when equipped with a slanted baffle. These oxidation ditches appear to be limited because the rotors must be spaced short distances apart in series around the channel so that they also operate as booster pumps. The rotors are manufactured by Ladeside Equipment Corporation, Passavant Corporation, Walker Process Division of CBI, and others.
Aeration discs that are 4.5 feet in diameter and 0.5 inches thick, made of perforated plastic and mounted on horizontally disposed shafts, rotate at about 60 rpm in oxidation ditches having multiple channels and are spaced short distances apart. These discs are manufactured by Envirex, Inc.
A floating perforated blade type aerator called the "OTA Aerator", is marketed by Cherne Industrial, Inc. It is 30 inches in diameter and 7 feet long. It has a variable speed hydraulic motor and can also be selectively submerged in order to balance oxygen transfer rate with oxygen demand and ditch velocity.
Jet aerators, such as directional-mix jet aerators manufactured by Pentech Division of Houdaille Industries, Inc., Cedar Falls, Iowa, can be used in deep oxidation ditches, such as a depth of 20 feet, and can thereby have comparatively high rates of oxygen transfer while using diffused or subsurface aeration. However, flow above the jet aerators is induced flow, not pumped flow, so that it is unaerated and later blends with the highly aerated liquor which is ejected from the jets. Moreover, backmixing of aerated liquor readily occurs as eddies develop above the jets.
Inflexibility of operation is characteristic of all prior art oxidation ditches because pumping and oxygen transfer are provided by the same device. Their pump/aerators are unable to provide for variable depth operation or flow equivalization in the oxidation ditch, i.e., a depth variation greater than one foot. In addition, they are unable to compensate for variations in temperature, inflow quantity, inflow BOD(5) content, inflow ammonia content, and mixed liquor suspended solids while maintaining adequate flow velocity and sufficient channel length in the anoxic zone for complete denitrification to occur within the channel so that a low effluent nitrate concentration is produced and so that rising sludge in the clarifier does not cause solids to overflow from the clarifier.
Of these variables, temperature appears to give the most frequent difficulty. During summertime operations, the microorganisms are very active and rapidly consume the available quantity of dissolved oxygen, but the C.sub.s level is much lower in summer than in winter so that dC/dt is markedly decreased. In consequence, the length of the aerobic zone tends to be quite short if the F/M ratio remains constant. Generally, a need exists for additional aerating capacity during hot weather.
In wintertime, particularly at subfreezing temperatures, pump/aerators of prior art oxidation ditches usually are unable to decrease aerating activity sufficiently while maintaining adequate mixing and propelling activity. Because the flow velocity must always be enough to prevent sedimentation in the channel, large amounts of dissolved oxygen are introduced into the aerobic zone, while the microorganisms are functioning slowly, so that the aerobic zone may extend throughout the endless channel, thereby making denitrification impossible. These effects may be minimized to some extent by increasing the MLSS levels during cold weather, but clarification also becomes less efficient because water becomes more dense as it cools, reaching maximum density at 4.degree. C. (37.degree. F.). The result is that smaller particles are likely to be carried with the effluent from the clarifier, thereby raising its biochemical oxygen demand, when MLSS levels are high during very cold weather.
All prior art oxidation ditches also lack a means to introduce air at a shallow depth to create an air-liquor mixture, and then to pump the air-liquor mixture to a depth greater than the floor or bottom of the channel, whereby energy can be saved because pumping a liquid consumes less energy than compressing a gas, and absorption of oxygen by water becomes greater as pressure increases. Further, all prior art oxidation ditches are unable to mix air with the liquor within the channel at a depth greater than the channel depth.
During intermittent in situ settling of the mixed liquor while using the entire oxidation channel as clarifier, all prior art oxidation ditches except the systems of Hungarian Pat. No. 166,160 and of German Pat. No. 2,300,273 have had no means to destroy the momentum of the mixed liquor, thereby enabling settling to commence quickly, except by reversing the pump/aerator, such as the rotors, as described by M. C. Goronszy in "Intermittent Operation of the Extended Aeration Process for Small Systems", Journal Water Pollution Control Federation, Volume 51, No. 2, Feb. 1979, pages 274-287.
The ideal design for both efficient oxygenation and denitrification is an oxidation ditch having a channel of sufficiently large cross section that the desired volume can be obtained within an oxic nitrification length plus an anoxic denitrification length that can be pumped with any desired number of pump/aerators which are disposed at a single location to provide point-source aeration, as contrasted to the multi-source aeration of the prior art for large oxidation ditches, while aerating homogenously, rather than heterogenously.
The parent applications, particularly Ser. Nos. 649,995, and now abandoned, 848,705, and now abandoned, and 28,383, now U.S. Pat. No. 4,278,547, have provided barrier oxidation ditches, utilizing submerged turbine pump/aerators, that satisfy these requirements. Each of these pump/aerators, as described in U.S. Pat. No. 4,260,486, comprises a pump motor operating a vertically mounted propeller shaft and propeller within a downdraft tube having a funnel at its upper end and a connection at its lower end to a J-shaped draft tube or deep oxygen contact duct which is rigidly embedded in the bottom or floor of the channel and which extends to a considerable depth, such as 15-40 feet, therebeneath. An air sparge is disposed immediately beneath the propeller. The discharge of the contact duct is oriented in the downstream direction and is separated from the funnel by a barrier which is sealably attached athwart the channel, so that all of the upstream mixed liquor in the intake channel must pass through the contact duct and none of the aerated downstream liquor in the discharge channel can backmix into the intake channel.
However, these deep duct pump/aerators have flexibility limitations. More specifically, when a pump/aerator motor is chosen by a designer for maximum efficiency at a selected set of conditions, no higher horsepower rating is used than is absolutely necessary. Any greater air sparging requirement than the designed amount is likely to cause the pump/aerator to "flood" (i.e., the impeller churns without pumping) or to require greater horsepower for pumping the liquor-air mixture of higher air content.
Removable contact duct diffusers are also selectively installed to deliver small bubbles of compressed air at approximately the deepest part of the deep contact duct for augmenting the air supply from the sparge,as particularly described in U.S. Pat. No. 4,260,486. When operating, such diffusers function as an air lift pump for the liquor being pumped through the deep duct and thereby accelerate the flow and assist the propeller. In effect, the diffusers and the sparge tend hydraulically to counterbalance each other so that use of the diffusers imparts additional flexibility of flow control as well as additional flexibility and capacity for aeration. On the other hand, use of the diffusers requires a compresser operating at a higher pressure than the compresser used for the sparge. In addition, the air added by the diffusers is in contact with the liquor being pumped for an appreciably shorter time than the air added by the sparge, so that mass transfer of difficultly absorbed oxygen is relatively less effective.
There is nevertheless a need for an additional aeration means which can add finely dispersed compressed air to the total liquor that is circulating in the channel without adding appreciably to the load on the propeller and without serious loss of contact time of bubbles with liquor within the deep contact duct, in order to obtain more aeration efficiency without hampering pumping efficiency. Moreover, there is a need for combining this additional aeration means with a means for preserving the kinetic energy in the flowing mixed liquor, in order to obtain more pumping efficiency. This flowing mixed liquor possesses momentum that could readily enable the liquor to flow past the funnel, were the barrier not present. When allowed to do so, it is termed induced flow. It is further desirable to provide a kinetic-energy preserving means that is capable of passing induced flow liquor through the deep contact duct with the directly pumped flow in order for the induced flow to become aerated by pressurized contact with dispersed air bubbles.
In engineering terms, it may be stated that experience in design and operation of barrier oxidation ditches indicates that maximum air sparge rates of 10.0 to 12.5 scfm/bhp are possible when the entire flow through the deep contact duct is pumped by and through the pump within the downdraft tube. In this 100% pumped flow design, the oxygen transfer rate is limited by the maximum air sparge rate that can be tolerated by the pump, up to the flood point of approximately 13-15 cfm/bhp. Therefore, there is a need to increase significantly the total flow rate through the deep contact duct per brake horsepower, thereby significantly increasing the air sparge rate and consequent oxygen transfer rate per brake horsepower per hour.